1. Field of Invention
The invention concerns a method for the production of sacks of tubular pieces which include plastic material, and a sack including plastic material. Furthermore, the invention concerns a device for the production of bottom cover sheet material, and a device for the production of sacks.
2. Description of the Prior Art
Sacks as well as production methods and devices have been known for a long time. In order to produce such sacks it is provided                that at least at one end area of a tubular piece a bottom rectangle is laid around a fold line to form triangular pockets,        that parts of the flaps of the bottom rectangle are folded back, and        that a bottom cover sheet is applied at least to areas of the flaps of the bottom rectangle and/or to areas of the triangular pockets and connected to them.        
The sacks provided with such bottoms are known as cross or block bottom sacks.
Many such sacks serve for the transport of bulk goods, such as perhaps building materials. Often used for this purpose are valve sacks which are manufactured separately and later filled via the valve. In order to achieve sufficient aeration of the as a rule powdery filling material in the sack, a sack wall is often perforated with fine needles. The hole diameters which can be achieved by needle punctures are however so large that along with air which is taken in, in particular during filling of the sack, an objectionable portion of the filling material also escapes from the holes and, for example, soils the sack on the outside.
Additional aeration concepts are therefore known in which the discharge of air is accomplished with a special form of aeration channel. For this the forming of special longitudinal seams is required. Such a sack is, for example, known from the Patent Application WO 2007/087965 A1 of the applicant. Such sacks have proven their value. However, in that process the tubular material from which the tubular pieces necessary for the production of the sacks are made individually must be produced from a web so that the introduction of the longitudinal seams is possible.
Particularly convenient but nonetheless high-strength sacks are often produced from a circular fabric which includes plastic threads. The circular fabric is as a rule coated on its outer side. Such sacks can be produced by hot air sealing without additional adhesive. In this case the coating of the fabric during the sealing process of the sack and cover sheet is melted on without damaging the fabric. In such a circular fabric the possibility of forming special longitudinal seams is not offered.
Furthermore, tubular sacks can be drawn upon as initial semi-finished products for the production of sacks which have been produced from a seamless tube produced in the blown-extrusion process. Also here no special longitudinal seams are formed. The production of such tubes is also economical.